The purpose of this project is to clarify the role of natural killer (NK) cells in host defense reactions against tumor cells. The project will deal with the following problems: Differential sensitivity of lymphomas that carry the same virally determined surface antigen complex to NK cells versus sensitized T cells. Attempts to overcome the low T-cell responsiveness of A/Sn mice to the YAC-lymphoma. One or several NK-sensitive target sites? Attempts to characterize the NK-sensitive target site. Physical characteristics of the mouse NK cell. Genetic control of NK-reactivity. Selection experiments at the somatic level (for NK cell resistance) and at the host level (for differential NK reactivity).